1. Technical Field
The present invention relates to an organic thin film transistor (TFT) and, more particularly, to an organic TFT which includes an organic acceptor film disposed between source and drain electrodes and an organic semiconductor film to obtain a doping effect.
2. Related Art
Since the development of polyacetylene, which is a conjugated organic polymer, intensive research for organic semiconductors has progressed in the fields of functional electronic and optical devices. Like other organic materials, organic semiconductors can be variously synthesized and easily molded into fibers or films. Also, organic semiconductors are highly flexible, conductive and economical. Organic thin film transistors (TFTs) of devices using conductive polymers include organic active films, and studies of organic TFTs began in 1980 and continue all over the world.
An organic TFT is structurally similar to a silicon TFT, except that a semiconductor active region is formed from an organic material instead of silicon. Compared to the silicon TFT, the organic TFT is simple to manufacture, economical, highly resistant to shock, and suitable for a substrate that is bent or folded. In particular, when an organic TFT is manufactured on a large area, the organic TFT is useful for products that require a low process temperature and need to be bent.
It is highly feasible that organic TFTs be used for driving devices of active matrix organic electro-luminescent display devices, smart cards, and plastic chips for smart tags or radio frequency identification (RFID). Thus, organic TFTs are now studied by many manufacturers, laboratories and colleges worldwide. The performance of an organic TFT depends on the capability of injecting carriers into an interface between a source/drain electrode and an organic semiconductor film.
Generally, the organic TFT is structurally similar to a silicon transistor. Like a field effect transistor (FET), the organic TFT operates on the principle that, when a voltage is applied to a gate, an electric field is applied to a gate insulating film. A current flowing through the organic TFT is obtained by applying a voltage between source and drain regions. In this case, the source region is grounded and supplies electrons or holes. The semiconductor active film located on the source and drain regions is an organic semiconductor film.
When no voltage is applied to the source and drain regions and the gate, charge is uniformly distributed throughout the semiconductor active film.
When a voltage lower than the threshold voltage of the TFT is applied to the gate, a current flows between drain and source in proportion on the applied voltage. If a voltage higher than the threshold voltage of the TFT (i.e., a positive voltage) is applied to the gate, positive charges (i.e., holes) are pushed upward due to an electric field caused by the applied voltage. As a result, a depletion film which includes no conductive charges is formed near a gate insulating film. In this case, when a voltage is applied between the source and drain regions, conductive charge carriers are reduced so that a current flows therebetween, and that current is smaller than when no voltage is applied to the gate.
On the contrary, if a voltage lower than the threshold voltage (i.e., a negative voltage) is applied to the gate, an electric field is generated in the gate insulating film. The electric field induces conductive charge carriers in the semiconductor active film, and the conductive charge carriers are accumulated between drain and source. The accumulated conductive charge carriers form the current channel between drain and source.
Therefore, the current flowing between the source and drain regions can be controlled by continuously applying a voltage therebetween, and by applying a positive or negative voltage to the gate. The ratio of the current when a positive voltage is applied to the current when a negative voltage is applied is referred to as the on/off ratio. The on/off ratio of an organic TFT is, preferably, as high as possible.
Various materials for forming the semiconductor active film of the organic TFT have been developed. The semiconductor active film maybe formed of organic semiconductors, such as pentacene, oligo-thiophene, poly(alkyl-thiophene), and poly(thienylenevinylene). Also, the organic semiconductor active film may be formed using vacuum deposition, preferably by thermal evaporation.
An organic TFT has the disadvantage of low charge mobility. After an FET including a pentacene thin film was developed by Brown et al. of Philips in 1995, Jackson et al. from Pennsylvania State University facilitated crystallization, and invented a transistor having a charge mobility of 1.5 cm2/Vs and an on/off ratio of about 108, which are equivalent to the characteristics of an a-Si:H FET. Pentacene, which consists of five benzene rings, is considered to be the most useful material that meets the requisite performance for a TFT.
A pentacene organic TFT exhibits the highest mobility among p-type semiconductors and realizes the same performance as an a-Si transistor. However, it is known that pentacene reacts with oxygen in the air, thus generating pentacenequinone. Once an organic semiconductor active film is oxidized, its coupling structure is broken, lowering charge mobility and causing lattice distortion of the interior of crystals. As a result, charge traps are formed so as to provoke charge scattering, which further deteriorates charge mobility.
Meanwhile, a method of doping a pentacene active film to enhance charge mobility was disclosed by Brown et al. of Philips. However, this method reveals a problem in that, although charge mobility increases with an increase in doping, the conductivity of an active film increases more than the charge mobility, thus reducing the on/off ratio. Accordingly, the foregoing doping method produces adverse effects, that is, an increase in the conductivity of the active film and a reduction in the on/off ratio.
Meanwhile, an organic light emitting device (OLED), which includes an organic film formed by depositing both an F4-TCNQ acceptor film and an amorphous TDATA, was proposed by Xiang Zhou et al. in Applied Physics Letters, Vol. 78, No. 4, 22 Jan. 2001. According to this paper, experimental results show that, as the OLED had a double or multiple layered structure including an amorphous TDATA and an F4-TCNQ acceptor film, current density increased, turn-on voltage decreased, and brightness improved.